PLC Mechanical Timing Relays

Mechanical timing relays are used to delay the opening or closing of contacts for circuit control. The operation of a mechanical timing relay is similar to that of a control relay, except that certain of its contacts are designed to operate at a preset time interval, after the coil is energized or de-energized.

Timers allow a multitude of operations in a control circuit to be automatically started and stopped at different time intervals.


the construction of an on-delay pneumatic (air) timer. The time-delay function depends on the transfer of air through a restricted orifice. The timedelay period is adjusted by positioning the needle valve to vary the amount of orifice restriction. When the coil is energized, the timed contacts are delayed from opening or closing. However, when the coil is de- energized, the timed contacts return instantaneously to their normal state.

This particular pneumatic timer has instantaneous contacts in addition to timed contacts. The instantaneous contacts change state as soon as the timer coil is powered while the delayed contacts change state at the end of the time delay.

Mechanical timing relays provide time delay through two arrangements. The first arrangement, on delay, provides time delay when the relay coil is energized. The second arrangement, off delay, provides time delay when the relay coil is de-energized.

On Delay:

The on-delay timer is sometimes referred to as DOE, which stands for delay on energize. The time delay of the contacts begins once the timer is switched on; hence the term on-delay timing.

Off Delay:

an off-delay timer circuit that uses a normally open, timed open (NOTO) contact. Off delay, provides time delay when the relay coil is de-energized